1. Field of the Invention
The present invention relates in general to measuring the humidity of a gas stream, and in particular, is directed to measuring the wet and dry bulb temperatures of a gas stream from a fossil fuel combustion process containing fly ash and other particulate matter.
2. Description of the Related Art
A psychrometer is a common device that measures the water content of air by simultaneously measuring the wet and dry bulb temperature. These values are used with a psychrometric chart to determine relative humidity.
There are numerous forms of psychrometers known in the prior art such as are disclosed in U.S. Pat. Nos. 4,175,436; 4,461,167; 4,222,261; 4,559,823; 4,625,550; and 4,409,834. There are also several commercial devices currently available for measuring the wet and dry bulb temperature of air. One such commercial device is the Assman psychrometer. This device uses a small blower to draw air across a pair of matched thermocouples of which one is wrapped with a saturated wick.
In the standard sling psychrometer, there are two thermometers, one for the wet bulb and one for the dry bulb. The wet bulb thermometer is wrapped with a porous material or wick and soaked in water. When it is whirled in air, evaporation from the wick causes the wet bulb thermometer to cool. At a steady state, the temperature with the wet bulb thermometer is known as a wet bulb temperature. For air/water systems, the wet bulb temperature is equal to the adiabatic saturation temperature. For combustion gases, these parameters differ by a few degrees.
While the foregoing devices function well for air/water systems, they are not suitable for a gas stream from a fossil fuel combustion process such as a coal combustion process. The flue gas contains fly ash and other particulates which would deposit onto and contaminate the wetted wick- causing incorrect measurements.
It is important to closely control and monitor flue gas humidity due to the recent interest in clean coal technology. Most of the current emission control technologies incorporate dry scrubbing or in-duct injection of an alkali based sorbent followed by flue gas humidification. Humidification increases overall sulfur oxide removal and further enhances particulate removal with an electrostatic precipitator. Consequently, it is necessary to operate a humidifier at a very low approach to adiabatic saturation temperature (T.sub.as). As a result, the likelihood of system failure increases from incomplete evaporation within the duct.
To avoid this type of failure, it is imperative that the amount of fluid injection be closely controlled and monitored. Fluid injection is controllable by a measurement of the degree of humidification of the treated gas. The instruments presently available for monitoring humidity of a flue gas employ expensive, sensitive optical and electronic sensors. These instruments are complicated and require periodic maintenance and calibration by skilled technicians.
Thus there is a need for an inexpensive, simple and low maintenance device for measuring the humidity of a dusty flue gas stream. The device needs to be constructed of materials that can withstand temperatures up to 400.degree. F. and is not sensitive to vibrations, weather, or corrosion. The device needs to make measurements which are primary measurements so as not to require calibration. The device must be constructed of readily available and inexpensive commercial equipment so as to facilitate repairs and construction.